I recall several years ago interviewing Stan Bulpitt, who did more than most people to get municipal leaf-composting going in the United States. He insists that soil building is at least 90 percent of gardening.

''When you follow a consistent program of composting, you can throw your concerns about soil pH and soil fertility out of the window,'' he said at the time. Mr. Bulpitt was referring to the concerns of many gardeners over countering excessive alkalinity or acidity in their soils and the need to add the right balance of plant nutrients.

My own experience since then has shown that statement to be absolutely true.

The Darien, Conn., nurseryman wasn't dismissing the importance of plant nutrients or a balanced soil pH. He simply didn't want gardeners to become agitated about them.

''Too many people believe gardeners have to be chemists in order to be successful. Yet the answer is so simple. If you compost consistently (he emphasizes the need for consistency), you will automatically bring about the right balance to your soils.''

What is soil? When we know the answer, soil-improvement methods are readily understood.

Good soil is composed of mineral fragments (weathered rock) and humus (decayed organic matter). Sir Albert Howard, who developed the famous Indore method of composting when he was an agriculture official in India early in this century, defines soil as ''fragments of mineral matter glued together by specks of organic matter provided by the activities of the invisible life of the soil.''

This invisible, or microscopic, soil life consistently processes spent organic matter into nutrients which the next generation of plants will take up.

Soil, then, is not a dead, inert substance but the home of teeming billions of microscopic plants and animals -- bacteria, actinomycetes, fungi, molds, yeasts, algae, and protozoa.

Most of these microbes live in the top 6 inches of soil, where their food (spent vegetation) is most plentiful. In an acre of organically rich soil these microbes can weigh as much as 600 pounds. Their life cycle is short and, as they die, their bodies contribute to the humus volume and nutrient value of the soil in a never-ending stream.

A vast majority of these microbes are beneficial to plants and animals; a few are not, but their numbers are consistently kept in check in healthy soil by the vigor of the beneficial population. Only when an imbalance occurs do plant problems surface.

These microbes, then, are the manufacturers of the food that feeds the plants which, in turn, feed us.

In at least one instance a fungus attaches itself to the plant and becomes an auxiliary root. Mycorrhiza (from myces, meaning fungus, and rhiza, root) absorbs some of its substance from the plant but more than makes up for this by absorbing soil nutrients and channeling them directly into the root system. When they die, these fungi are absorbed into the plant as a food.

More visible in the soil are the earthworms which pull dead organic matter down into the soil, partly digest it, and make it more readily subject to microbial processing. Soil ingested by earthworms and then excreted is several times richer than the surrounding soil in available plant nutrients.

On its way through the earthworm, microbial life in the soil is increased dramatically as well. Earthworm tunnels allow air and moisture to get down into the soil. Plant roots frequently follow the earthworm tunnel, feeding off the rich mucus the earthworm has used to line the tunnel. In short, they are most beneficial to the gardener.

The gardener's unpaid labor force is how Sir Albert Howard describes the earthworms and soil microbes.

All these soil organisms combined form the ''digestive system'' of the soil. It follows then that the more of them there are, the more readily organic matter is processed into plant nutrients. And the more organic matter they are fed, the more their numbers increase.

If organic matter -- leaves, straw, kitchen waste, spent plants, etc. -- are dug or tilled into nutrient-poor soil that has lost much of its humus, it will not readily digest the material. During the resulting slow decay process, plants growing in the area will actually be deprived of nutrients, principally nitrogen. The decay organisms use this limited supply of nutrients as fuel during their attack on the organic waste.

Eventually, the decay process will be complete and all those borrowed nutrients will be available once again along with newly processed nutrients. In the interim, however, plants may go a little hungry.

Adding a balanced fertilizer, preferably organic, rock powders, or aged manure, will help, but the best approach is to add compost directly to the soil. Compost is organic material that has been predigested (in the compost pile) before being added to the soil. As such it provides plant nutrients, humus, and a microbial population to the soil, all in one shot.

Sir Albert's classic composting method involves layering garden and kitchen waste with manure and soil. Six inches of garden waste is laid down and covered with 2 to 3 inches of manure, followed by a thin layer of soil (Sir Albert recommended a one-eighth-inch thickness) to which has been added a sprinkling of lime or wood ashes if the soil is acid.

This process is repeated as materials become available, until the heap has reached its desired size. If no manure is available, you can sprinkle fish meal, bone meal, or feather meal as a substitute. The heap should be moist, but not so wet that water could be squeezed from it. Water the pile if the weather is hot and dry. It's best to build the compost heap in the shade.

Three weeks after the heap has been finished it should be turned, then turned again about 5 weeks later. Four weeks after the final turn the compost is ready. At this stage it is not fully broken down, but it has been digested enough to go onto the land.

It has since been discovered that the more frequently compost is turned, the more rapidly it breaks down. Mechanically breaking down the organic materials into small pieces, as with a shredder, also greatly speeds up decomposition.

Still another composting secret is to have a pile large enough to build up and hold its own heat. Large heaps, however, aren't always possible for the home gardener.

Thus, it's a big help to place the materials in a bin that allows the air to get in, yet protects the smaller volume of compost from the cold, drying effects of the wind. Commercial models now are available. Another option is to make a circle out of fencing wire and line it with several thicknesses of paper or corrugated cardboard. Punch holes through the paper to allow for aeration.

It is said that composting is more of an art than a science. This is true. Your own experience and observations will soon enable you to develop a composting system that best suits your needs.

Meanwhile, as you add compost to your garden soil, it will steadily build up its ability to digest whatever organic material you give it. A farmer in Kenya once wrote to Sir Albert, saying that composting had so improved his plantation soil that it ''would eat anything thrown at it from a gunny bag to corn stover.''

Once the microbial population of the soil has reached a high level, organic materials -- leaves, hay, garden waste, and the like -- can be plowed in directly without it ever needing to be predigested in a compost heap. Compost, however, is always an outstanding garden product.

I add a trowelful of compost to every seedling I set out as well as sprinkle compost lightly over all the seeds I sow. For the rest, shredded leaves go on the garden as a mulch. All season long the soil life eats up the mulch from below, processing it into good plant food. By year's end what had begun as 2 to 3 inches of shredded leaves is perhaps an eighth of an inch thick. Where it disappears altogether I add more mulch.

The idea is to never leave the soil exposed to the hot rays of the sun or to let the soil life go hungry.